In cellular networks, such as a 3GPP (Third Generation Partnership Project) mobile network, various types of wireless communication devices may be used. Such a wireless communication device may for example be user equipment (UE), a mobile phone, a smartphone, a data modem, a mobile computer, or another kind of terminal device.
The present disclosure is described within the context of Long Term Evolution (LTE), i.e. Evolved Universal Terrestrial Radio Access Network (UMTS) Mobile Telecommunications System (E-UTRAN). It should be understood that the problems and solutions described herein are equally applicable to wireless access networks and wireless communication devices implementing other access technologies and standards. LTE is used as an example technology where the embodiments are suitable, and using LTE in the description therefore is particularly useful for understanding the problem and solutions solving the problem.
For ease of understanding, LTE Mobility is described in the following.
Radio Resource Control (RRC) (Third Generation Partnership Project (3GPP) Technical Specification (TS) 36.331, e.g. V10.8.5 (2013-01)) is the main signaling protocol for configuring, re-configuring and general connection handling in the LTE radio access network (E-UTRAN). RRC controls many functions such as connection setup, mobility, measurements, radio link failure and connection recovery. These functions are of relevance for the present disclosure, and are therefore described in some further detail below.
A wireless communication device in LTE can be in two RRC states: RRC_CONNECTED and RRC_IDLE. In RRC_CONNECTED state, mobility is network-controlled based on e.g. measurements provided by the wireless communication device. I.e. the network decides when and to which cell an UE should be handed over, based on e.g. measurements provided by the wireless communication device. The network, i.e. the LTE radio base station (called evolved Node Base station (eNodeB or eNB), respectively, in E-UTRAN) configures various measurement events, thresholds etc. based on which the wireless communication device then sends reports to the network, such that the network can make a wise decision to hand over the wireless communication device to a stronger cell as the wireless communication device moves away from the present cell.
Dual connectivity is one of the features being standardized within the umbrella work of small cell enhancements within 3GPP Rel-12. Dual connectivity is a feature that allows a wireless communication device to simultaneously receive and transmit to at least two different network points. The two different network points are usually denoted as master eNodeB (MeNB) and secondary eNodeB (SeNB). MeNBs serve a master cell group (MCG), and SeNBs serve a secondary cell group (SCG). It is assumed that the radio resource control (RRC) protocol, which is responsible for configuring the wireless communication device, is terminated within the MeNB. While the wireless communication device receives RRC control signaling via the MCG, it may receive user data via both MCG and SCG.
In the split bearer architecture option of dual connectivity the downlink data is split on the Packet Data Convergence Protocol (PDCP) layer in the MeNB. The MeNB may route PDCP Packet Data Units (PDUs) dynamically via MeNB Radio Link Control (RLC) to the wireless communication device directly, or via an internode interface, also known as backhaul channel, to the SeNB and SeNB RLC to the wireless communication device. The data flow from MeNB to SeNB via the internode interface is typically controlled by a flow control protocol, in order to balance the SeNB buffer fill state. For this purpose flow control feedback had been defined in 3GPP TS 36.425.
However, there is a need for techniques which allow for balancing transmission of data between an access node and a wireless communication device, particularly in a case in which the wireless communication device may be connected via two separate links with a cellular network.